On Fri, Oct 31, 2025 at 8:44 AM KaFai Wan <[email protected]> wrote: > > When conditional jumps are performed on the same scalar register > (e.g., r0 <= r0, r0 > r0, r0 < r0), the BPF verifier incorrectly > attempts to adjust the register's min/max bounds. This leads to > invalid range bounds and triggers a BUG warning. > > The problematic BPF program: > 0: call bpf_get_prandom_u32 > 1: w8 = 0x80000000 > 2: r0 &= r8 > 3: if r0 > r0 goto <exit> > > The instruction 3 triggers kernel warning: > 3: if r0 > r0 goto <exit> > true_reg1: range bounds violation u64=[0x1, 0x0] s64=[0x1, 0x0] u32=[0x1, > 0x0] s32=[0x1, 0x0] var_off=(0x0, 0x0) > true_reg2: const tnum out of sync with range bounds u64=[0x0, > 0xffffffffffffffff] s64=[0x8000000000000000, 0x7fffffffffffffff] > var_off=(0x0, 0x0) > > Comparing a register with itself should not change its bounds and > for most comparison operations, comparing a register with itself has > a known result (e.g., r0 == r0 is always true, r0 < r0 is always false). > > Fix this by: > 1. Enhance is_scalar_branch_taken() to properly handle branch direction > computation for same register comparisons across all BPF jump operations > 2. Adds early return in reg_set_min_max() to avoid bounds adjustment > for unknown branch directions (e.g., BPF_JSET) on the same register > > The fix ensures that unnecessary bounds adjustments are skipped, preventing > the verifier bug while maintaining correct branch direction analysis. > > Reported-by: Kaiyan Mei <[email protected]> > Reported-by: Yinhao Hu <[email protected]> > Closes: > https://lore.kernel.org/all/[email protected]/ > Signed-off-by: KaFai Wan <[email protected]> > --- > kernel/bpf/verifier.c | 33 +++++++++++++++++++++++++++++++++ > 1 file changed, 33 insertions(+) > > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c > index 542e23fb19c7..a571263f4ebe 100644 > --- a/kernel/bpf/verifier.c > +++ b/kernel/bpf/verifier.c > @@ -15995,6 +15995,8 @@ static int is_scalar_branch_taken(struct > bpf_reg_state *reg1, struct bpf_reg_sta > > switch (opcode) { > case BPF_JEQ: > + if (reg1 == reg2) > + return 1; > /* constants, umin/umax and smin/smax checks would be > * redundant in this case because they all should match > */ > @@ -16021,6 +16023,8 @@ static int is_scalar_branch_taken(struct > bpf_reg_state *reg1, struct bpf_reg_sta > } > break; > case BPF_JNE: > + if (reg1 == reg2) > + return 0; > /* constants, umin/umax and smin/smax checks would be > * redundant in this case because they all should match > */ > @@ -16047,6 +16051,12 @@ static int is_scalar_branch_taken(struct > bpf_reg_state *reg1, struct bpf_reg_sta > } > break; > case BPF_JSET: > + if (reg1 == reg2) { > + if (tnum_is_const(t1)) > + return t1.value != 0; > + else > + return (smin1 <= 0 && smax1 >= 0) ? -1 : 1; > + } > if (!is_reg_const(reg2, is_jmp32)) { > swap(reg1, reg2); > swap(t1, t2); > @@ -16059,48 +16069,64 @@ static int is_scalar_branch_taken(struct > bpf_reg_state *reg1, struct bpf_reg_sta > return 0; > break; > case BPF_JGT: > + if (reg1 == reg2) > + return 0; > if (umin1 > umax2) > return 1; > else if (umax1 <= umin2) > return 0; > break; > case BPF_JSGT: > + if (reg1 == reg2) > + return 0;
This is uglier than the previous version. reg1 == reg2 is a syzbot territory. We shouldn't uglify the code everywhere because of it. pw-bot: cr
